1. Field of the Invention
This invention relates to a method for recovering an oxidation catalyst used for the preparation of terephthalic acid by the liquid-phase oxidation of an alkylbenzene such as para-xylene with a gas containing molecular oxygen from the residue of the reaction mother liquor, and more specifically, to a method for recovering a heavy metal catalyst such as manganese or cobalt as a carbonate precipitate having purity and good filterability.
2. Description of the Prior Art
Commercial methods for preparing terephthalic acid which are now widely accepted involve the liquid-phase oxidation of alkylbenzenes such as para-xylene with a gas containing molecular oxygen in a lower aliphatic monocarboxylic acid as a solvent in the presence of a catalyst containing a heavy metal such as cobalt, manganese, or a bromine compound. Terephthalic acid is separated from the reaction mixture resulting from such a liquid-phase oxidation reaction by means such as filtration or centrifugal separation, and then the reaction solvent is recovered from the reaction mother liquor by means such as distillation. The residue resulting after the removal of the solvent contains the heavy metal catalyst. In order to utilize expensive metals such as cobalt and manganese effectively, it is important to reuse the heavy metal catalyst. In addition to the heavy metal catalyst, however, the residue of the mother liquor contains great quantities of unreacted alkylbenzene, and organic impurities such as 4-carboxybenzaldehyde and para-toluic acid as reaction intermediates and unidentified oxidation by-products.
Furthermore, since acetic acid, for example, is used as the solvent under high temperature and pressure conditions, and a halogen such as bromine is also used as a catalyst component in the above liquid-phase oxidation reaction, the reaction liquor is corrosive. While such a reaction liquor circulates within the system, corrosion of the equipment takes place, and metals such as iron, chromium, nickel, molybdenum or copper contained in the materials of the equipment contaminate the reaction liquor. In addition, the terephthalic acid which is sparingly soluble in the solvent deposits on various parts of the manufacturing equipment and on the inside walls of the pipes and causes blockage. In order to prevent such blockage, the equipment is washed with an aqueous solution of an alkali such as sodium hydroxide, and this often results in the contamination of the reaction liquor with the alkali metal. It is known that all of these impurities impede the oxidation reaction of alkylbenzenes and cause a reduction in the quality and yield of terephthalic acid. Accordingly, when the oxidation reaction catalyst is recovered from the residue of the mother liquor for recycling, it is important to prevent impurities such as the organic impurities, the heavy metal impurities and the alkali metal from contaminating the heavy metal catalyst recovered. Various methods have therefore been proposed to recover the oxidation catalyst without contamination by various impurities.
For example, U.S. Pat. No. 2,964,559 discloses a method which comprises extracting the residue of the mother liquor containing tar-like by-products with a solvent such as water, a lower aliphatic carboxylic acid or a mixture thereof to thereby recover a heavy metal catalyst such as cobalt or manganese as an extract not containing the tar-like by-products. U.S. Pat. No. 3,341,470 discloses a method of recovering the heavy metal catalyst without contamination with tar-like organic by-products and metal impurities such as iron, chromium or copper contained in the residue of the reaction mother liquor. Specifically, the method comprises incinerating the residue of the reaction mother liquor to thereby oxidize the heavy metal catalyst, dissolving the catalyst in a mineral acid such as sulfuric acid, adding sodium sulfide or hydrogen sulfide, etc. to the solution to remove any copper contaminant in the form of a copper sulfide precipitate, then diluting the solution with water, neutralizing the diluted solution with a hydroxide to a pH of about 4, adding calcium carbonate to the solution to separate metal impurities such as iron or chromium as their oxide precipitates, adding sodium carbonate to the solution from which the metal impurities have been removed, to thereby obtain cobalt or manganese as a carbonate precipitate, and finally treating the precipitate with an organic carboxylic acid to recover cobalt or manganese as the organic carboxylate.
Furthermore, Japanese Patent Publication 14339/71 discloses a method comprising: extracting the residue of the reaction mother liquor containing oxidation reaction by-products and iron with water as an extracting solvent to separate the insoluble solid impurities, adding an alkali to the aqueous solution containing the extracted oxidation catalyst to adjust the pH of the aqueous solution to 4 to 5 and separating the tar-like by-products as a float and the iron as a basic iron acetate precipitate, and thereafter adding an alkali metal such as sodium carbonate to recover the cobalt or manganese as a carbonate precipitate carbonate.
Japanese Laid-Open Patent Publication 34088/72 discloses a method comprising: extracting the residue of the reaction mother liquor with water to separate insoluble solid impurities, adding an alkali to the aqueous extract to adjust the pH of the aqueous extract to 6.0 to 6.8, either separating iron and lead as a sparingly soluble terephthalate or oxidizing the iron compound in the aqueous extract to a trivalent iron compound and thereafter adjusting the pH of the aqueous extract to 4.0 to 6.8 to separate iron or lead as a sparingly soluble terephthalate, and adding a carbonate compound such as sodium carbonate to the resulting filtrate to recover cobalt or manganese as the carbonate.
The procedure of adding an alkali metal or carbonate to the aqueous extract obtained by extracting the residue of the mother liquor containing the heavy metal catalyst with water thereby to recover the heavy metal catalyst as a carbonate precipitate, which is employed in many of the prior techniques illustrated above, is considered to be a superior method capable of recovering the heavy metal catalyst in a highly pure form without contamination with impurities detrimental to the oxidation reaction.
However, according to this procedure, the precipitated particles of the heavy metal carbonate such as cobalt carbonate or manganese carbonate are recovered as very fine amorphous colloidal particles. Accordingly, long periods of time are required to separate the precipitate of the heavy metal carbonate from the precipitate-containing solution and to wash the separated precipitate with water. Thus, for commercial operation, this separating apparatus requires enormous filtering and washing areas. The formation of precipitated particles which can be easily filtered and washed has been regarded as an industrially important problem.
One measure for coping with this problem is disclosed, for example, in Japanese Laid-Open Patent Publication 1553/72 which discloses a method wherein an alkali metal carbonate is added to the heavy metal(cobalt)-containing solution extracted from the residue of the mother reaction liquor with water at a temperature of not less than 170.degree.F (77.degree.C), preferably at least 195.degree.F (91.degree.C), to thereby precipitate the cobalt as a carbonate having improved filterability and washability. However, the precipitated particles obtained by this method are still unsatisfactory from the standpoint of filterability and washability. Furthermore, since the precipitation of the carbonate in this method is performed at an elevated temperature, the alkali metal carbonate abruptly decomposes, evolving carbon dioxide which forms bubbles insoluble in the solution. This not only leads to the loss of the alkali metal carbonate, but also results in a marked expansion of the volume of the solution, which in turn requires a tank of increased capacity for forming the precipitate.
As stated above, no advantageous method has yet been discovered for recovering a heavy metal catalyst such as cobalt or manganese used in the liquid-phase oxidation of alkylbenzenes to produce terephthalic acid from the mother liquor residue as a carbonate precipitate having good filterability and washability while efficiently removing the organic impurities or heavy metal impurities such as chromium, iron, nickel or copper, all of which are detrimental to the oxidation reaction.